Efficient Singlet Fission Discovered in a Disordered Acene Film

Substitution effects on the photoinduced excited state dynamics of perylenemonoimides in solution and thin films

Perylene monoimide (PMI) derivatives are attracting significant attention due to their strong absorption in the visible range, thermal stability, and synthetic accessibility. These properties make them promising for application in various areas such as optoelectronic devices, photosensitizers, etc. In this work, the photophysical properties and excited state dynamics of four different PMI derivatives (PMIB, BrPMITB, PMITB, and APITB) were studied in solution and thin films utilizing steady-state and time-resolved spectroscopic techniques. Among the four PMI derivatives, APITB is designed as a donor-acceptor dyad, with thianthrene as a donor and PMI as an acceptor. The activation of the triplet state through the spin-orbit charge transfer intersystem crossing (SOCT-ISC) process in THF was observed upon substitution with the thianthrene group at the peri position of the PMI moiety. The SOCT-ISC process facilitates triplet formation in the APITB dyad within 423 ps. Meanwhile, other PMI derivatives showed fluorescence within the femtosecond timescale in THF. The PMI derivatives in thin films displayed different photo physical properties to those in THF. This discrepancy arises due to the effective intermolecular coupling between the PMI derivatives in thin films.

Investigation of ultrafast intermediate states during singlet fission in lycopene H-aggregate using femtosecond stimulated Raman spectroscopy

The singlet fission process involves the conversion of one singlet excited state into two triplet states, which has significant potential for enhancing the energy utilization efficiency of solar cells. Carotenoid, a typical π conjugated chromophore, exhibits specific aggregate morphologies known to display singlet fission behavior. In this study, we investigate the singlet fission process in lycopene H-aggregates using femtosecond stimulated Raman spectroscopy aided by quantum chemical calculation. The experimental results reveal two reaction pathways that effectively relax the S2 (11Bu+) state populations in lycopene H-aggregates: a monomer-like singlet excited state relaxation pathway through S2 (11Bu+) → 11Bu- → S1 (21Ag-) and a dominant sequential singlet fission reaction pathway involving the S2 (11Bu+) state, followed by S* state, a triplet pair state [1(TT)], eventually leading to a long lifetime triplet state T1. Importantly, the presence of both anionic and cationic fingerprint Raman peaks in the S* state is indicative of a substantial charge-transfer character.

Computational Discovery of Intermolecular Singlet Fission Materials Using Many-Body Perturbation Theory

Intermolecular singlet fission (SF) is the conversion of a photogenerated singlet exciton into two triplet excitons residing on different molecules. SF has the potential to enhance the conversion efficiency of solar cells by harvesting two charge carriers from one high-energy photon, whose surplus energy would otherwise be lost to heat. The development of commercial SF-augmented modules is hindered by the limited selection of molecular crystals that exhibit intermolecular SF in the solid state. Computational exploration may accelerate the discovery of new SF materials. The GW approximation and Bethe-Salpeter equation (GW+BSE) within the framework of many-body perturbation theory is the current state-of-the-art method for calculating the excited-state properties of molecular crystals with periodic boundary conditions. In this Review, we discuss the usage of GW+BSE to assess candidate SF materials as well as its combination with low-cost physical or machine learned models in materials discovery workflows. We demonstrate three successful strategies for the discovery of new SF materials: (i) functionalization of known materials to tune their properties, (ii) finding potential polymorphs with improved crystal packing, and (iii) exploring new classes of materials. In addition, three new candidate SF materials are proposed here, which have not been published previously.

Controlling Interchromophore Coupling in Diamantane-Linked Pentacene Dimers To Create a "Binary" Pair

Two isomeric pentacene dimers, each linked by a diamantane spacer, have been synthesized. These dimers are designed to provide experimental evidence to support quantum mechanical calculations, which predict the substitution pattern on the carbon-rich diethynyldiamantane spacer to be decisive in controlling the interpentacene coupling. Intramolecular singlet fission (i-SF) serves as a probe for the existence and strength of the electronic coupling between the two pentacenes, with transient absorption spectroscopy as the method of choice to characterize i-SF. 4,9-Substitution of diamantane provides a pentacene dimer (4,9-dimer) in which the two chromophores are completely decoupled and that, following photoexcitation, deactivates to the ground state analogous to a monomeric pentacene chromophore. Conversely, 1,6-substitution provides a pentacene dimer (1,6-dimer) that exhibits sufficiently strong coupling to drive i-SF, resulting in correlated triplet M(T1T1) yields close to unity and free triplet (T1 + T1) yields of ca. 50%. Thus, the diamantane spacer effectively switches "on" or "off" the coupling between the chromophores, based on the substitution pattern. The binary control of diamantane contrasts other known molecular spacers designed only to modulate the coupling strength between two pentacenes.

Impact of Packing Geometry on Excimer Characteristics and Mobility in Perylene Bisimide Polycrystalline Films

Efficient exciton transport is essential for high-performance optoelectronics. Considerable efforts have been focused on improving the exciton mobility in organic materials. While it is feasible to improve mobility in organic systems by forming well-ordered stacks, the formation of trap states, particularly the lower-lying states referred to as excimers, remains a significant challenge to enhancing mobility. The mobility of excimer excitons intricately depends on the strength of excitonic coupling in terms of Förster-type diffusive exciton transfer processes. Given that the formation and mobility of excimer excitons are highly sensitive to molecular arrangements (packing geometries), conducting comprehensive investigations into the structure-property relationship in organic systems is crucial. In this study, we prepared three types of polycrystalline films of perylene bisimide (PBI) by varying substituents at the imide and bay positions, which allowed us to tailor the properties of excimer excitons and their mobility based on packing geometries and excitonic coupling strengths. By utilizing femtosecond transient absorption spectroscopy, we observed ultrafast excimer formation in the higher coupling regime, while in the lower coupling regime, the transition from Frenkel to excimer excitons occurs with a time constant of 500 fs. Under high pump-fluence, exciton-exciton annihilation processes occur, indicating the diffusion of excimer excitons. Intriguingly, employing a three-dimensional diffusion model, we derived a diffusion constant that is 3000 times greater in the high coupling regime than in the low coupling regime. To investigate the optoelectronic properties in the form of a bulk system, we fabricated n-type organic field effect transistors and obtained 8000 times higher mobility in the high coupling regime. Furthermore, photocurrent measurements enable us to investigate the charge carrier transport by mobile excimer excitons, suggesting a 230-fold improvement in external quantum efficiency with tightly packing PBI molecules compared to the low coupling regime. These findings not only offer valuable insights into optimizing organic materials for optoelectronic devices but also unveil the intriguing potential of exciton migration within excimers.

An Exploration of Substituent Effects on the Photophysical Properties of Monobenzopentalenes

Monobenzopentalenes have received moderate attention compared to dibenzopentalenes, yet their accessibility as stable, non-symmetric structures with diverse substituents could be interesting for materials applications, including molecular photonics. Recently, monobenzopentalene was considered computationally as a potential chromophore for singlet fission (SF) photovoltaics. To advance this compound class towards photonics applications, the excited state energetics must be characterized, computationally and experimentally. In this report we synthesized a series of stable substituted monobenzopentalenes and provided the first experimental exploration of their photophysical properties. Structural and opto-electronic characterization revealed that all derivatives showed 1H NMR shifts in the olefinic region, bond length alternation in the pentalene unit, low-intensity absorptions reflecting the ground-state antiaromatic character and in turn the symmetry forbidden HOMO-to-LUMO transitions of ~2 eV and redox amphotericity. This was also supported by computed aromaticity indices (NICS, ACID, HOMA). Accordingly, substituents did not affect the fulfilment of the energetic criterion of SF, as the computed excited-state energy levels satisfied the required E(S1)/E(T1)>2 relationship. Further spectroscopic measurements revealed a concentration dependent quenching of the excited state and population of the S2 state on the nanosecond timescale, providing initial evidence for unusual photophysics and an alternative entry point for singlet fission with monobenzopentalenes.

Structural Disorder at the Edges of Rubrene Crystals Enhances Singlet Fission

Materials that undergo singlet fission are of interest for their use in light-harvesting, photocatalysis, and quantum information science, but their ability to undergo fission can be sensitive to local variations in molecular packing. Herein we employ transient absorption microscopy, molecular dynamics simulations, and electronic structure calculations to interrogate how structures found at the edges of orthorhombic rubrene crystals impact singlet fission. Within a micrometer-scale spatial region at the edges of rubrene crystals, we find that the rate of singlet fission increases nearly 4-fold. This observation is consistent with formation of a region at crystal edges with reduced order that accelerates singlet fission by disrupting the symmetry found in rubrene's orthorhombic crystal structure. Our work demonstrates that structural distortions of singlet fission materials can be used to control fission in time and in space, potentially offering a means of controlling this process in light harvesting and quantum information applications.

Re-Thinking Dimer Design Principles with Indolonaphthyridine Intramolecular Singlet Fission

Singlet fission is a phenomenon that could significantly improve the efficiency of photovoltaic devices. Indolonaphthyridine thiophene (INDT) is a photostable singlet fission material that could potentially be utilised in singlet fission-based photovoltaic devices. This study investigates the intramolecular singlet fission (i-SF) mechanism of INDT dimers linked via para-phenyl, meta-phenyl and fluorene bridging groups. Using ultra-fast spectroscopy the highest rate of singlet fission is found in the para-phenyl linked dimer. Quantum calculations show the para-phenyl linker encourages enhanced monomer electronic coupling. Increased rates of singlet fission were also observed in the higher polarity o-dichlorobenzene, relative to toluene, indicating that charge-transfer states have a role in mediating the process. The mechanistic picture of polarisable singlet fission materials, such as INDT, extends beyond the traditional mechanistic landscape.

Singlet Fission in Perylene Monoimide Single Crystals and Polycrystalline Films

Singlet fission (SF) is a spin-allowed process in which a photogenerated singlet exciton down-converts into two triplet excitons. Perylene-3,4-dicarboximide (PMI) has singlet and triplet state energies of 2.4 and 1.1 eV, respectively; thus making SF slightly exoergic and providing triplet excitons that have sufficient energy to raise the efficiency of single-junction solar cells by reducing thermalization losses from hot excitons formed when absorbed photons have energies higher than the semiconductor bandgap. However, PMI SF in the solid state has not been studied previously. Here, we show that 2,5-diphenyl-N-(2-ethylhexyl)perylene-3,4-dicarboximide (dp-PMI) crystallizes into a slip-stacked intermolecular morphology favorable for SF. Transient absorption microscopy and spectroscopy show that dp-PMI SF occurs in ≤50 ps in both single crystals and polycrystalline thin films with a triplet yield of 150 ± 20%. Ultrafast SF in the solid state, the high triplet yield, and its photostability make dp-PMI an attractive candidate for SF-enhanced solar cells.

Singlet fission preserves polarisation correlation of excitons

Singlet fission (SF) holds the promise to circumvent the photovoltaic efficiency limit to reach a power-conversion efficiency above 34%. SF of TIPS-pentacene (TIPS-Pn) has been investigated but its mechanism is yet to be well elucidated. Recently, we developed a nanoparticle (NP) system, in which doping of TIPS-Pn in a host matrix yields a range of average intermolecular distances, d, to study the dependence of SF in TIPS-Pn on d. At large d values, where the bimolecular SF process should be unfavourable, a relatively high SF quantum yield (Φ_SF) is still observed, which implies a deviation from a random distribution of TIPS-Pn throughout the NP. Here, using polarisation-sensitive femtosecond time-resolved spectroscopy and Monte Carlo simulations of exciton migration and SF, we quantify the level of clustering of TIPS-Pn in the host matrix, which is responsible for the higher than expected Φ_SF. The experimental data indicate a preservation of polarisation correlation by SF, which is uncommon because energy transfer in amorphous materials tends to result in depolarisation. We show that the preservation of polarisation correlation is due to SF upon exciton migration. Although exciton migration decorrelates polarisation, SF acts to remove decorrelated excitons to give an overall preservation of polarisation correlation.

Singlet Fission in Colloidal Nanoparticles of Amphipathic Diketopyrrolopyrrole Derivatives: Probing the Role of the Charge Transfer State

To evaluate the role of the charge transfer (CT) state in the singlet fission (SF) process, we prepared three 3,6-bis(thiophen-2-yl)diketopyrrolopyrrole (TDPP) derivatives with zero (Ph₂TDPP), one (Ph₂TDPP-COOH), and two (Ph₂TDPP-(COOH)₂) carboxylic groups, respectively. Their colloidal nanoparticles were also prepared by a simple precipitation method. The SF dynamics and mechanism in these colloid nanoparticles were investigated by using steady-state/transient absorption and fluorescence spectroscopy. Steady-state absorption spectra reveal that the strength of the CT resonance interactions between the adjacent DPP units is increased gradually from Ph₂TDPP to Ph₂TDPP-COOH and then to Ph₂TDPP-(COOH)₂. Fluorescence and transient absorption spectra demonstrate that SF is proceeded via a CT-assisted superexchange mechanism in these three nanoparticles. Furthermore, SF rate and yield are enhanced gradually with the increase of the number of the carboxylic group, which may be attributed to the enhancement of the CT coupling strength. The result of this work not only provides a better understanding of the SF mechanism especially for the role of the CT state but also gives some new insights for the design of efficient SF materials based on DPP derivatives.

Effects of the Separation Distance between Two Triplet States Produced from Intramolecular Singlet Fission on the Two-Electron-Transfer Process

To harvest two triplet excitons of singlet fission (SF) via a two-electron transfer efficiently, the revelation of the key factors that influence the two-electron-transfer process is necessary. Here, by using steady-state and transient absorption/fluorescence spectroscopy, we investigated the two-electron-transfer process from the two triplet excitons of intramolecular SF (iSF) in a series of tetracene oligomers (dimer, trimer, and tetramer) with 7,7,8,8-tetracyanoquinodimethane (TCNQ) as an electron acceptor in solution. Quantitative two-electron transfer could be conducted for the trimer and tetramer, and the rate for the tetramer is faster than that for the trimer. However, the maximum efficiency of the two-electron transfer in the dimer is relatively low (∼47%). The calculation result of the free energy change (ΔG) of the second-electron transfer for these three compounds (-0.024, -0.061, and -0.074 eV for the dimer, trimer, and tetramer, respectively) is consistent with the experimental observation. The much closer ΔG value to zero for the dimer should be responsible for its low efficiency of the two-electron transfer. Different ΔG values for these three oligomers are attributed to the different Coulomb repulsive energies between the two positive charges generated after the two-electron transfer that is caused by their various intertriplet distances. This result reveals for the first time the important effect of the Coulomb repulsive energy, which depends on the intertriplet distance, on the two-electron transfer process from the two triplet excitons of iSF.

Pitfalls of quantifying intersystem crossing rates in singlet-fission chromophore solutions

Singlet fission (SF), a process that produces two triplet excitons from one singlet exciton, has attracted recent interest for its potential to circumvent the detailed-balance efficiency limit of single-junction solar cells. For the potential of SF to be fully realized, accurate assignment and quantification of SF is necessary. Intersystem crossing (ISC) is another process of singlet to triplet conversion that is important to distinguish from SF to avoid either over- or under-estimation of SF triplet production. Here, we quantify an upper bound on the rate of ISC in two commonly studied SF chromophores, TIPS-pentacene and TIPS-tetracene, by using transient absorption spectroscopy of solutions of varying concentrations in toluene. We show that SF in solutions of these acenes has previously been misidentified as ISC, and vice versa. By determining a bimolecular SF rate constant in concentrated solutions in which SF dominates over ISC, we distinguish triplet formation due to SF from triplet formation due to ISC, and show that the characteristic time scale of ISC must be longer than 325 ns in TIPS-pentacene, while it must be longer than 118 ns in TIPS-tetracene. We additionally note that no excimer formation is observed in the relatively dilute (up to 8 mM) solutions studied here, indicating that previous excimer formation observed at much higher concentrations may be partially due to aggregate formation. This work highlights that an accurate quantification of ISC is crucial as it leads to accurate determination of SF rate constants and yields.

Mechanism of Singlet Fission in Carotenoids from a Polyene Model System

Singlet fission (SF) is the process of formation of multiple excitons (triplet) from a locally excited singlet state. The mechanism of SF in polyacenes has been shown to proceed via a charge transfer intermediate state. However, carotenoids are not understood in the context of SF. This is possibly due to the complicated multireference nature of the low-lying excited states of carotenoids and the presence of a dark 2¹A_g state below the optically bright ¹B_u state. In this work, we show that the dark A_g state in polyenes and/or carotenoids, along with the charge transfer states, plays a pivotal role in the SF process. We notice that the relative importance of these states varies with a change in geometry and the overall presence of multiple pathways is crucial to the success of the SF process in carotenoid aggregates and disordered geometries.

Effect of nanoscale confinement on ultrafast dynamics of singlet fission in TIPS-pentacene

Singlet fission (SF) is a phenomenon for the generation of a pair of triplet excitons from a singlet excited molecule interacting with another adjacent molecule in its ground electronic state. By increasing the effective number of charge carriers and reducing thermal dissipation of excess energy, SF is promised to enhance light-harvesting efficiency for photovoltaic applications. While SF has been extensively studied in thin films and crystals, the same has not been explored much within a confined medium. Here, we report the ultrafast SF dynamics of triisopropylsilylethynyl pentacene (TIPS-Pn) in micellar nanocavity of varying sizes (prepared from TX-100, CTAB, and SDS surfactants). The nanoparticle with a smaller size contains weakly coupled chromophores and is shown to be more efficient for SF followed by triplet generation as compared to the nanoparticles of larger size which contain strongly coupled chromophores and are less efficient due to the presence of singlet exciton traps. Through these studies, we delineate how a subtle interplay between short-range and long-range interaction among chromophores confined within nanoparticles, fine-tuned by the curvature of the micellar interface but irrespective of the nature of the micelle (cationic or anionic or neutral), play a crucial role in SF through and generation of triplets.

Excited-State Dynamics of 5,14- vs 6,13-Bis(trialkylsilylethynyl)-Substituted Pentacenes: Implications for Singlet Fission

Singlet fission is a process in conjugated organic materials that has the potential to considerably improve the performance of devices in many applications, including solar energy conversion. In any application involving singlet fission, efficient triplet harvesting is essential. At present, not much is known about molecular packing arrangements detrimental to singlet fission. In this work, we report a molecular packing arrangement in crystalline films of 5,14-bis(triisopropylsilylethynyl)-substituted pentacene, specifically a local (pairwise) packing arrangement, responsible for complete quenching of triplet pairs generated via singlet fission. We first demonstrate that the energetic condition necessary for singlet fission is satisfied in amorphous films of the 5,14-substituted pentacene derivative. However, while triplet pairs form highly efficiently in the amorphous films, only a modest yield of independent triplets is observed. In crystalline films, triplet pairs also form highly efficiently, although independent triplets are not observed because triplet pairs decay rapidly and are quenched completely. We assign the quenching to a rapid nonadiabatic transition directly to the ground state. Detrimental quenching is observed in crystalline films of two additional 5,14-bis(trialkylsilylethynyl)-substituted pentacenes with either ethyl or isobutyl substituents. Developing a better understanding of the losses identified in this work, and associated molecular packing, may benefit overcoming losses in solids of other singlet fission materials.

Singlet Fission Materials for Photovoltaics: from Small Molecules to Macromolecules

Singlet fission (SF) is a spin-allowed process in which a singlet state splits into two triplet states. Materials that enable SF have attracted great attention in the last decade, mainly stemming from the potential of overcoming the Shockley-Queisser (SQ) limit in photoenergy conversion. In the past decade, a large number of new molecules exhibiting SF have been explored and many devices based on SF materials have been studied, though the mechanistic understanding is still obscure. This review focuses on the recent developments of SF materials, including small molecules, oligomers and polymers. The molecular design strategies and related mechanisms of SF are discussed. Then the dynamics of charge transfer and energy transfer between SF materials and other materials are introduced. Further, we discuss the progresses of implementing SF in photovoltaics. It is hoped that a comprehensive understanding to the SF materials, devices and mechanism may pave a new way for the design of next generation photovoltaics. This article is protected by copyright. All rights reserved.

Enhancing Singlet Fission Coupling with Nonbonding Orbitals

Singlet fission (SF) is a process where a singlet exciton is split into a pair of triplet excitons. The increase in the excitonic generation can be exploited to enhance the efficiency of solar cells. Molecules with conjugated π bonds are commonly developed for optoelectronic applications including SF, due to their low energy gaps. The electronic coupling for SF in such well-stacked π-conjugated molecule pairs can be rather limited due to the orthogonal π and π* orbital overlaps that are involved in the coupling elements, leading to a large cancellation in the coupling. In the present work, we show that such limits can be removed by involving triplet states of different origins, such as those with nonbonding n orbitals. We demonstrate such an effect for formaldehyde and methylenimine dimers, with a low-lying n-π* triplet state (T₁) in addition to the π-π* triplet (T₂). We show that the coupling can be enhanced by 40 times or more for the formaldehyde dimer, and 15 times or more for the methylenimine dimer, with the T₁-T₂ state as the end product of SF. With 1759 randomly oriented pairs of formaldehyde derived from a molecular dynamics simulation, the coupling from a singlet exciton to this T₁-T₂ state is, on an average, almost two times larger than that for a regular T₁-T₁ state. We investigated a few families that have been shown to be prospective candidates for SF, using our proposed strategy. However, our unfavorable results indicate that there are clear difficulties in fulfilling the E_S1 ≳ E_T1 + E_T2 energy criterion. Nevertheless, our results provide a new molecular design concept for better SF (and triplet-triplet annihilation, TTA) materials that allows future development.

Singlet fission and triplet pair recombination in bipentacenes with a twist

We investigate triplet pair dynamics in pentacene dimers that have varying degrees of coplanarity (pentacene-pentacene twist angle). The fine-tuning of the twist angle was achieved by alternating connectivity at the 1-position or 2-positions of pentacene. This mix-and-match connectivity leads to tunable twist angles between the two covalently linked pentacenes. These twisted dimers allow us to investigate the subtle effects that the dihedral angle between the covalently linked pentacenes imparts on singlet fission and triplet pair recombination dynamics. We observe that as the dihedral angle between the two bonded pentacenes is increased, the rates of singlet fission decrease, while the accompanying decrease in triplet recombination rates is stark. Temperature-dependent transient optical studies combined with theoretical calculations show that the triplet pair recombination proceeds primarily through a direct multiexciton internal conversion process. Calculations further show that the significant decrease in recombination rates can be directly attributed to a corresponding decrease in the magnitude of the nonadiabatic coupling between the singlet multiexcitonic state and the ground state. These results highlight the importance of the twist angle in designing systems that exhibit rapid singlet fission, while maintaining long triplet pair lifetimes in pentacene dimers.

Vacancy control in acene blends links exothermic singlet fission to coherence

The fission of singlet excitons into triplet pairs in organic materials holds great technological promise, but the rational application of this phenomenon is hampered by a lack of understanding of its complex photophysics. Here, we use the controlled introduction of vacancies by means of spacer molecules in tetracene and pentacene thin films as a tuning parameter complementing experimental observables to identify the operating principles of different singlet fission pathways. Time-resolved spectroscopic measurements in combination with microscopic modelling enables us to demonstrate distinct scenarios, resulting from different singlet-to-triplet pair energy alignments. For pentacene, where fission is exothermic, coherent mixing between the photoexcited singlet and triplet-pair states is promoted by vibronic resonances, which drives the fission process with little sensitivity to the vacancy concentration. Such vibronic resonances do not occur for endothermic materials such as tetracene, for which we find fission to be fully incoherent; a process that is shown to slow down with increasing vacancy concentration.

Resolving electron injection from singlet fission-borne triplets into mesoporous transparent conducting oxides

Photoinduced electron transfer into mesoporous oxide substrates is well-known to occur efficiently for both singlet and triplet excited states in conventional metal-to-ligand charge transfer (MLCT) dyes. However, in all-organic dyes that have the potential for producing two triplet states from one absorbed photon, called singlet fission dyes, the dynamics of electron injection from singlet vs. triplet excited states has not been elucidated. Using applied bias transient absorption spectroscopy with an anthradithiophene-based chromophore (ADT-COOH) adsorbed to mesoporous indium tin oxide (nanoITO), we modulate the driving force and observe changes in electron injection dynamics. ADT-COOH is known to undergo fast triplet pair formation in solid-state films. We find that the electronic coupling at the interface is roughly one order of magnitude weaker for triplet vs. singlet electron injection, which is potentially related to the highly localized nature of triplets without significant charge-transfer character. Through the use of applied bias on nanoITO:ADT-COOH films, we map the electron injection rate constant dependence on driving force, finding negligible injection from triplets at zero bias due to competing recombination channels. However, at driving forces greater than -0.6 eV, electron injection from the triplet accelerates and clearly produces a trend with increased applied bias that matches predictions from Marcus theory with a metallic acceptor.

Free-triplet generation with improved efficiency in tetracene oligomers through spatially separated triplet pair states

Singlet fission (SF) can potentially boost the efficiency of solar energy conversion by converting a singlet exciton (S₁) into two free triplets (T₁ + T₁) through an intermediate state of a correlated triplet pair (TT). Although efficient TT generation has been recently realized in many intramolecular SF materials, their potential applications have been hindered by the poor efficiency of TT dissociation. Here we demonstrate that this can be overcome by employing a spatially separated ¹(T…T) state with weak intertriplet coupling in tetracene oligomers with three or more chromophores. By using transient magneto-optical spectroscopic methods, we show that free-triplet generation can be markedly enhanced through the SF pathway that involves the spatially separated ¹(T…T) state rather than the pathway mediated by the spatially adjacent TT state, leading to a marked improvement in free-triplet generation with an efficiency increase from 21% for the dimer to 85% (124%) for the trimer (tetramer).

The Role of the Core Attachment Positioning in Triggering Intramolecular Singlet Exciton Fission in Perylene Diimide Tetramers

Previous studies have proposed that the presence of a flexible π-bridge linker is crucial in activating intramolecular singlet exciton fission (iSEF) in multichromophoric systems. In this study, we report the photophysical properties of three analogous perylene diimide (PDI) dendritic tetramers showing flexible/twisted π-bridged structures with α- and β-substitutions and a rigid/planar structure with a β-fused ring (βC) connection to a benzodithiophene-thiophene (BDT-Th) core. The rigidity and enhanced planarity of βC lead to significant intramolecular charge transfer and triplet formation via an intersystem crossing pathway. Steady-state spectroscopic measurements reveal similar absorption and emission spectra for the α-tetramer and the parent PDI monomer. However, their fluorescence quantum yield is significantly different. The negligible fluorescence yield of the α-tetramer (0.04%) is associated with a competitive nonradiative decay pathway. Indeed, for this twisted compound in a high polar environment, a fast and efficient iSEF with a triplet quantum yield of 124% is observed. Our results show that the α-single-bond connections in the α compound are capable of interrupting the coupling among the PDI units, favoring iSEF. We propose that the formation of the double triplet (¹[TT]) state is through a superposition of singlet states known as [S₁S₀]_[TT]CT, which has been suggested previously for pentacene derivatives. Using steady-state and time-resolved spectroscopic experiments, we demonstrate that the conformational flexibility of the linker itself is necessary but not sufficient to allow iSEF. For the case of the other twisted tetramer, β, the strong π-π co-facial interactions between the adjacent PDI units in its structure lead to excimer formation. These excimer states trap the singlet excitons preventing the formation of the ¹[TT] state, thus inhibiting iSEF.

Singlet exciton fission in a modified acene with improved stability and high photoluminescence yield

We report a fully efficient singlet exciton fission material with high ambient chemical stability. 10,21-Bis(triisopropylsilylethynyl)tetrabenzo[a,c,l,n]pentacene (TTBP) combines an acene core with triphenylene wings that protect the formal pentacene from chemical degradation. The electronic energy levels position singlet exciton fission to be endothermic, similar to tetracene despite the triphenylenes. TTBP exhibits rapid early time singlet fission with quantitative yield of triplet pairs within 100 ps followed by thermally activated separation to free triplet excitons over 65 ns. TTBP exhibits high photoluminescence quantum efficiency, close to 100% when dilute and 20% for solid films, arising from triplet-triplet annihilation. In using such a system for exciton multiplication in a solar cell, maximum thermodynamic performance requires radiative decay of the triplet population, observed here as emission from the singlet formed by recombination of triplet pairs. Combining chemical stabilisation with efficient endothermic fission provides a promising avenue towards singlet fission materials for use in photovoltaics.

Designing optimised molecules for singlet fission is crucial to improve the efficiency of solar cells beyond its theoretical limit. Here, the authors investigate pentacene derivative TTBP, which exhibits high stability and luminescence yield, and find it highly suitable for exciton multiplication purposes.

Ultrafast spectroscopy reveals singlet fission, ionization and excimer formation in perylene film

Singlet exciton fission (SF) is a spin-allowed process whereby two triplet excitons are created from one singlet exciton. This phenomenon can offset UV photon energy losses and enhance the overall efficiency in photovoltaic devices. For this purpose, it requires photostable commercially available SF materials. Excited state dynamics in pure perylene film, ease of commercial production, is studied by time-resolved fluorescence and femtosecond transient absorption techniques under different photoexcitation energies. In film, polycrystalline regions contain perylene in H-type aggregate form. SF takes place from higher excited states of these aggregates in ultrafast time scale < 30 fs, reaching a triplet formation quantum yield of 108%. Moreover, at λ_ex = 450 nm singlet fission was detected as a result of two-quantum absorption. Other competing relaxation channels are excimer (1 ps) and dimer radical cation formation (< 30 fs). Excimer radiatively relaxes within 19 ns and radical cation recombines in 3.2 ns. Besides, exciton self-trapping by crystal lattice distortions occurs within hundreds of picosecond. Our results highlight potential of simple-fabricated perylene films with similar properties as high-cost single crystal in SF based photovoltaic applications.

Correlated Triplet Pair Formation Activated by Geometry Relaxation in Directly Linked Tetracene Dimer (5,5'-Bitetracene)

Singlet fission (SF) materials have the potential to overcome the traditional external quantum efficiency limits of organic light-emitting diodes (OLEDs). In this study, we theoretically designed an intramolecular SF molecule, 5,5'-bitetracene (55BT), in which two tetracene units were directly connected through a C-C bond. Using quantum chemical calculation and the Fermi golden rule, we show that 55BT undergoes efficient SF induced by geometry relaxation in a locally excited singlet state, ¹(S₀S₁). Compared with another high-performing SF system, the tetracene dimer in the crystalline state, 55BT has advantages when used in doped systems owing to covalent bonding of the two tetracene units. This feature makes 55BT a promising candidate triplet sensitizer for near-infrared OLEDs.

Singlet fission from upper excited singlet states and polaron formation in rubrene film

Femtosecond fluorescence up-conversion and transient absorption pump-probe setups are applied to study the relaxation dynamics of the lower and upper excited singlet electronic states in easy-to-make rubrene films. Upon 250 nm (4.96 eV) excitation, singlet fission was observed directly from S2 state bypassing S1 state within 30 fs i.e. breaking the classical Kasha rule. From the transient absorption measurements, polaron formation was also detected on the same time scale. Both singlet fission and polaron formation are accelerated from upper excited states compared with S1 state. Our work shows that rubrene films with low degree of crystallinity could display efficient singlet fission, notably in the case of excitation to upper lying electronic states. This can strongly expand the applications of rubrene in organic electronics. Moreover, our results will provide a new direction for synthesizing novel materials with optimized excited state properties for organic photovoltaic applications.

Planarity and Length of the Bridge Control Rate and Efficiency of Intramolecular Singlet Fission in Pentacene Dimers

Singlet fission (SF) improves the power conversion efficiency of optoelectronic devices by converting high-energy photons into two triplet excitons. SF dynamics and efficiency (Φ) are controlled by various factors. Here, the effect of planarity and length of the bridge in pentacene dimers on the intramolecular SF (iSF) process was investigated by synthesizing the dimers connected by bridges having fluorene (FL-PD, planar), methyl-substituted biphenyl (MBP-PD, twisted), and diphenyl acetylene (DPA-PD, longer) groups and characterizing their excited-state relaxation dynamics using nanosecond and femtosecond pump-probe spectroscopy. Transient absorption studies reveal that iSF dynamics of FL-PD having a planar bridge are ∼787 times faster (187 ps) and exhibit higher Φ (198%) by feasible electronic coupling, compared to MBP-PD possessing a twisted bridge showing a low Φ of ∼16%. However compared to FL-PD, iSF dynamics of DPA-PD with an increase of bridge length are slower by an order (1.09 ns) and show comparable Φ of 185% through extended conjugation. Thus, the planarity and length of the bridge in pentacene dimers control the rate and efficiency of the iSF process.

Singlet fission relevant energetics from optimally tuned range-separated hybrids

As a promising idea to design high-efficiency organic photovoltaics, singlet fission (SF) mechanism, i.e., generating two triplet excitons out of a single photon absorption, has recently come into the spotlight. Even though much effort has been devoted to this arena, accurately accounting for the SF process from the theoretical perspective has proven to be challenging. Herein, the SF energetics have thoroughly been investigated with the help of optimally tuned range-separated hybrid functionals (OT-RSHs) in both gas and solvent phases. Taking a series of experimentally known SF chromophores as working models, we have proposed and validated several variants of OT-RSH approximations for the reliable prediction of the energy levels which match the crucial criteria for the SF process, namely, the negative singlet-triplet and triplet-triplet energy gaps. We scrutinize the role of the OT-RSH ingredients, i.e., the underlying density functional approximations, short- and long-range exact-like exchange, as well as the range-separation parameter, for our purpose. The newly designed OT-RSHs outperform the standard RSHs and other related schemes such as screened-exchange approximations as well as other density functionals from different rungs for describing the SF energetics. More importantly, it is unveiled that although the OT-RSH coupled with the polarizable continuum model, OT-RSH-PCM, as well as the screened versions, OT-SRSHs, which account for the screening effect by the electron correlation through the scalar dielectric constant have some advantages over gas-phase computations using OT-RSHs, the energetics criteria of the SF process may not necessarily be satisfied. This in turn corroborates the idea of performing both the optimal tuning procedure and subsequent computations of the SF relevant energetics using OT-RSHs as a more reliable and affordable framework, at least for the present purpose. The applicability of the proposed models is also put into broader perspective, where they are used for the computational design of several chromophores as promising candidates prone to utilization in the SF-based materials. Hopefully, our recommended OT-RSHs can function as efficient models for both the theoretical modeling of SF chromophores and confirming the experimental observations in the field.

Direct, Mediated, and Delayed Intramolecular Singlet Fission Mechanism in Donor-Acceptor Copolymers

Donor-acceptor (D-A) extended copolymers have shown great potential to be exploited for intramolecular singlet fission (iSF) because of their modular tunability and intrinsic ability to incorporate low-lying charge-transfer (CT) and a triplet-pair (¹TT) states. While the SF mechanism has been widely debated in homo- and heterodimers, little is known about the singlet splitting process in A-D-A copolymer trimers. Unlike traditional two-site SF, the process of iSF in D-A copolymers involves three molecular units consisting of two A's and one D following an A-D-A polymeric chain. This scenario is, therefore, different from that of the homodimer analogues in terms of which states (if any) may drive the SF process. In this work, we identify how singlet splitting occurs in prototypical iSF D-A copolymer poly(benzodithiophene-alt-thiophene-1,1-dioxide) by means of wave packet propagations on the basis of the linear vibronic coupling model Hamiltonian. Our results reveal that three different mechanisms drive the S₁ → ¹TT population transfer via antisymmetric and symmetric vibrational motion, including two favorable mechanisms of direct and mediated interactions, as well as a parasitic decay pathway that potentially delays the process. Remarkably, we uncover the interplay between an upper state of marked multiexcitonic character and a low-lying CT state in balancing the splitting efficiency, which anticipates their major role in defining future guidelines for the molecular design of D-A copolymers for iSF.

Bridge Resonance Effects in Singlet Fission

A major benefit of intramolecular singlet fission (iSF) materials, in which through-bond interactions mediate triplet pair formation, is the ability to control the triplet formation dynamics through molecular engineering. One common design strategy is the use of molecular bridges to mediate interchromophore interactions, decreasing electronic coupling by increasing chromophore-chromophore separation. Here, we report how the judicious choice of aromatic bridges can enhance chromophore-chromophore electronic coupling. This molecular engineering strategy takes advantage of "bridge resonance", in which the frontier orbital energies are nearly degenerate with those of the covalently linked singlet fission chromophores, resulting in fast iSF even at large interchromophore separations. Using transient absorption spectroscopy, we investigate this bridge resonance effect in a series of pentacene and tetracene-bridged dimers, and we find that the rate of triplet formation is enhanced as the bridge orbitals approach resonance. This work highlights the important role of molecular connectivity in controlling the rate of iSF through chemical bonds and establishes critical design principles for future use of iSF materials in optoelectronic devices.

Activating intramolecular singlet exciton fission by altering π-bridge flexibility in perylene diimide trimers for organic solar cells

In this study, two analogous perylene diimide (PDI) trimers, whose structures show rotatable single bond π-bridge connection (twisted) vs. rigid/fused π-bridge connection (planar), were synthesized and investigated. We show via time resolved spectroscopic measurements how the π-bridge connections in A-π-D-π-A-π-D-π-A multichromophoric PDI systems strongly affect the triplet yield and triplet formation rate. In the planar compound, with stronger intramolecular charge transfer (ICT) character, triplet formation occurs via conventional intersystem crossing. However, clear evidence of efficient and fast intramolecular singlet exciton fission (iSEF) is observed in the twisted trimer compound with weaker ICT character. Multiexciton triplet generation and separation occur in the twisted (flexible-bridged) PDI trimer, where weak coupling among the units is observed as a result of the degenerate double triplet and quintet states, obtained by quantum chemical calculations. The high triplet yield and fast iSEF observed in the twisted compound are due not only to enthalpic viability but also to the significant entropic gain allowed by its trimeric structure. Our results represent a significant step forward in structure-property understanding, and may direct the design of new efficient iSEF materials.

Supertransport of excitons in atomically thin organic semiconductors at the 2D quantum limit

Long-range and fast transport of coherent excitons is important for the development of high-speed excitonic circuits and quantum computing applications. However, most of these coherent excitons have only been observed in some low-dimensional semiconductors when coupled with cavities, as there are large inhomogeneous broadening and dephasing effects on the transport of excitons in their native states in materials. Here, by confining coherent excitons at the 2D quantum limit, we first observed molecular aggregation-enabled 'supertransport' of excitons in atomically thin two-dimensional (2D) organic semiconductors between coherent states, with a measured high effective exciton diffusion coefficient of ~346.9 cm2/s at room temperature. This value is one to several orders of magnitude higher than the values reported for other organic molecular aggregates and low-dimensional inorganic materials. Without coupling to any optical cavities, the monolayer pentacene sample, a very clean 2D quantum system (~1.2 nm thick) with high crystallinity (J-type aggregation) and minimal interfacial states, showed superradiant emission from Frenkel excitons, which was experimentally confirmed by the temperature-dependent photoluminescence (PL) emission, highly enhanced radiative decay rate, significantly narrowed PL peak width and strongly directional in-plane emission. The coherence in monolayer pentacene samples was observed to be delocalised over ~135 molecules, which is significantly larger than the values (a few molecules) observed for other organic thin films. In addition, the supertransport of excitons in monolayer pentacene samples showed highly anisotropic behaviour. Our results pave the way for the development of future high-speed excitonic circuits, fast OLEDs, and other optoelectronic devices.

Delocalization effects in singlet fission: Comparing models with two and three interacting molecules

We present surface hopping simulations of singlet fission in 2,5-bis(fluorene-9-ylidene)-2,5-dihydrothiophene (ThBF). In particular, we performed simulations based on quantum mechanics/molecular mechanics (QM/MM) schemes in which either two or three ThBF molecules are inserted in the QM region and embedded in their MM crystal environment. Our aim was to investigate the changes in the photodynamics that are brought about by extending the delocalization of the excited states beyond the minimal model of a dimer. In the simulations based on the trimer model, compared to the dimer-based ones, we observed a faster time evolution of the state populations, with the largest differences associated with both the rise and decay times for the intermediate charge transfer states. Moreover, for the trimer, we predicted a singlet fission quantum yield of ∼204%, which is larger than both the one extracted for the dimer (∼179%) and the theoretical upper limit of 200% for the dimer-based model of singlet fission. Although our study cannot account for the effects of extending the delocalization beyond three molecules, our findings clearly indicate how and why the singlet fission dynamics can be affected.

Lessons from intramolecular singlet fission with covalently bound chromophores

Molecular dimers, oligomers, and polymers are versatile components in photophysical and optoelectronic architectures that could impact a variety of applications. We present a perspective on such systems in the field of singlet fission, which effectively multiplies excitons and produces a unique excited state species, the triplet pair. The choice of chromophore and the nature of the attachment between units, both geometrical and chemical, play a defining role in the dynamical scheme that evolves upon photoexcitation. Specific final outcomes (e.g., separated and uncorrelated triplet pairs) are being sought through rational design of covalently bound chromophore architectures built with guidance from recent fundamental studies that correlate structure with excited state population flow kinetics.

Topology of quantum coherence networks in singlet fission: mapping exciton states into real space and the dislocation induced three dimensional manifolds

An understanding of the global structure of quantum coherence networks in coupled multistate systems is of great importance for the development of emerging quantum technologies such as quantum control and quantum materials design. Here, we study the topology of a quantum coherence network of a typical singlet exciton fission system by mapping the exciton states into crystal structures in real space. The defects in crystals could lead to changes in the topological structures, and also fission dynamics. In particular, we demonstrate that the dislocation induced three dimensional manifold, which differs from its lower dimensional counterparts globally, could generate exotic global structures, such as chiral spirals, and modulate singlet fission substantially. The findings may shed light on the new possibilities of engineering effective structures for fission materials.

Impact of t-butyl substitution in a rubrene emitter for solid state NIR-to-visible photon upconversion

Solid state NIR-to-visible photon upconversion (UC) mediated by triplet-triplet annihilation (TTA) is necessitated by numerous practical applications. Yet, efficient TTA-UC remains a highly challenging task. In this work palladium phthalocyanine-sensitized NIR-to-vis solid UC films based on a popular rubrene emitter are thoroughly studied with the primary focus on revealing the impact of t-butyl substitution in rubrene on the TTA-UC performance. The solution-processed UC films were additionally doped with a small amount of emissive singlet sink tetraphenyldibenzoperiflanthene (DBP) for collecting upconverted singlets from rubrene and in this way diminishing detrimental singlet fission. Irrespective of the excitation conditions used, t-butyl-substituted rubrene (TBR) was found to exhibit enhanced TTA-UC performance as compared to that of rubrene at an optimal emitter doping of 80 wt% in polystyrene films. Explicitly, in the TTA dominated regime attained at high excitation densities, 4-fold higher UC quantum yield (ΦUC) achieved in TBR-based films was caused by the reduced fluorescence concentration quenching mainly due to suppressed singlet fission. Under low light conditions, i.e. in the regime governed by spontaneous triplet decay, even though triplet exciton diffusion was obstructed in TBR films by t-butyl moieties, the subsequently reduced TTA rate was counterbalanced by both suppressed singlet fission and non-radiative triplet quenching, still ensuring higher ΦUC of these films as compared to those of unsubstituted rubrene films.

Impact of non-equilibrium molecular packings on singlet fission in microcrystals observed using 2D white-light microscopy

Singlet fission, the process of splitting a singlet exciton into two triplet excitons, has been proposed as a mechanism for improving the efficiency of future photovoltaic devices. In organic semiconductors exhibiting singlet fission, the geometric relationship between molecules plays an important role by setting the intermolecular couplings that determine the system energetics. Here, we spatially image TIPS-pentacene microcrystals using ultrafast two-dimensional white-light microscopy and discover a low-energy singlet state sparsely distributed throughout the microcrystals, with higher concentrations at edges and morphological defects. The spectra of these singlet states are consistent with slip-stacked molecular geometries and increased charge-transfer couplings. The picosecond-timescale kinetics of these low-energy singlet states matches that of the correlated triplet-pair state, which we attribute to singlet/triplet-pair interconversion at these sites. Our observations support the conclusion that small populations of geometries with favourable energetics can play outsized roles in singlet fission processes.

Singlet Fission Rate: Optimized Packing of a Molecular Pair. Ethylene as a Model

A procedure is described for unbiased identification of all π-electron chromophore pair geometry choices that locally maximize the rate of conversion of a singlet exciton into a singlet biexciton (triplet pair), using a simplified version of the diabatic frontier orbital model of singlet fission (SF). The resulting approximate optimal geometries provide insight and are expected to represent useful starting points for searches by more advanced methods. The general procedure is illustrated on a pair of ethylenes as the simplest model of a π-electron system, but it is applicable to pairs of much larger molecules, with dozens of non-hydrogen atoms, and not necessarily planar. We first examine the value of |T^A|², the square of the electronic matrix element for SF with initial excitation fully localized on partner A, on a grid of several billion geometries within the six-dimensional space of physically realizable possibilities. Several of the optimized pair geometries are somewhat unexpected, but all are found to follow the qualitative guidance proposed earlier. In the neighborhood of each local maximum of |T^A|², consideration of mixing with charge-transfer configurations and of excitonic interaction between partners A and B determines the SF energy balance and yields squared matrix elements |T*|² and |T**|² for the lower and upper excitonic states S* and S**, respectively. Assuming Boltzmann populations of these states, the geometry is further optimized to maximize k, the sum of the SF rates obtained from Marcus theory, and this reorders the suitable geometries substantially. At 87 pair geometries, the |T*|² and |T**|² values are compared with those obtained from high-level ab initio nonorthogonal configuration interaction calculations and found to follow the same trend. Finally, the biexciton binding energy at the optimized geometries is calculated. Altogether, 13 significant local maxima of SF rate for a pair of ethylenes are identified in the physically relevant part of space that avoids molecular interpenetration in the hard-sphere approximation. The three best geometries are twist-stacked, slip-stacked, and L-shaped. The maxima occur at the (five-dimensional) surfaces of seven six-dimensional "parent" regions of space centered at physically inaccessible geometries at which the calculated SF rate is very large but the two ethylenes interpenetrate. The results are displayed in interactive graphics. The computer code ("Simple") written for these calculations is flexible in that it permits a choice of performing the search for local maxima in six dimensions on |T^A|², |T*|², or k. It is available as freeware at https://cloud.uochb.cas.cz/simple .